So as to improve the luminance of surface radiation type semiconductor light emitting elements such as an LED, there have been recently developed double heterostructure semiconductor light emitting elements wherein a Bragg light reflecting layer has been disposed between a pn junction and a substrate such that the light emitted at the pn junction which goes toward the substrate is reflected by the Bragg light reflecting layer toward a light output surface so that the light is efficiently radiated from the output surface.
FIG. 7 shows a cross section of a conventional semiconductor light emitting element as described, wherein H is a semiconductor light emitting element composed of a plate Bragg light reflecting layer 2, the first conductivity type semiconductor cladding layer 3, a semiconductor active layer 4, and the second conductivity type semiconductor cladding layer 5 successively formed on the first conductivity type semiconductor substrate 1, all of which having a thickness of the order of from 0.01 to several dozen .mu.m.
In the semiconductor light emitting element H, a pn junction X to be a light emitter is formed by the first conductivity type semiconductor cladding layer 3 and the semiconductor active layer 4, and an upper electrode 6 is formed on the surface A of the second conductivity type semiconductor cladding layer 5 (i.e. light output surface); a lower electrode 7 is formed beneath the substrate 1.
A flow of driving current between the upper electrode 6 and the lower electrode 7 starts the emission of light at the pn junction X. The light heading toward the light output surface A is radiated from the output surface A. The light heading toward the substrate 1 is reflected back to the light output surface A by the plate light reflecting layer 2, and also radiated from the output surface A.
Consequently, luminance of the semiconductor light emitting element H can be improved, since the light emitted at the pn junction X is not absorbed in the substrate 1, but can be radiated from the clement.
However, the light heading toward the light output surface A is absorbed in the upper electrode 6 on the output surface A, which reduces the amount of light to be radiated from the light emitting element, thus resulting in failure to sufficiently improve the luminance of the semiconductor light emitting element H.
As such, there is still a limitation in improving the luminance of a semiconductor light emitting element of this structure having a light reflecting layer interposed between the substrate and the pn junction, and the development of a light emitting element with higher luminance has been desired. While many attempts have been made to incorporate various improvements so as to meet the need of the market, none of them have ever successfully achieved the goal of providing a light emitting element with a higher luminance.
Accordingly, an object of the invention is to provide a semiconductor light emitting element permitting efficient output of the light emitted at the pn junction, whereby to increase the luminance of a semiconductor light emitting element such as an LED.